THE STEM 171 



petuated in the early development of the secondary organization, 

 since the radially disposed elements make their 'first appearance 

 opposite the clusters of primary wood, and only as the second- 

 ary segments widen out later do they finally bridge over the 

 intervals in the continuity of the cylinder. On the upper side 

 a branch causes a still more prominent hiatus in the inner region 

 of the cylinder. The condition in a calamitean stem may next 

 engage our attention. Here the primary wood is much less well 

 developed even than in the later Sigillariae, and only in the 

 earliest representatives of the genus does it show an indication 

 of the ancestral centripetal type of development. The task of 

 bridging over the primary intervals by secondary growth is cor- 

 respondingly greater than in the sigillarian stem, and the bays 

 extending into the secondary wood proportionately deeper and 

 wider. In the upper region of the cylinder is figured a siphonostelic 

 branch which is responsible for a wide hiatus in the secondary 

 cylinder. An inspection of a, b, and c makes it clear that the 

 progressive degeneracy of the primary wood as well as the depar- 

 ture of traces belonging to the branches causes interruptions 

 in the continuity of the secondary wood. These are filled by soft 

 tissues continuous with the parenchyma of the pith. The second- 

 ary cylinder is characterized, as has been demonstrated in the 

 diagrams, by bands of radial parenchyma which are often wrongly 

 called medullary rays. That they cannot receive this appellation 

 with any accuracy will be apparent from a comparison of the 

 conditions presented by a, b, and c. In a the rays in no case reach 

 the pith. In b the radial parenchyma is clearly continuous with 

 the soft tissues of the pith in the intervals between the isolated 

 segments of primary wood, while in those regions of the secondary 

 cylinder subtended by primary xylem, which of course represent 

 the primitive topographical relation, no such continuity is possible. 

 In c, by the still further reduction of the primary structures of the 

 wood, a much larger number of bands of radial parenchyma appear 

 to abut on the pith. It is clear from the conditions outlined in the 

 diagram under discussion that it is entirely inappropriate to de- 

 scribe the radial storage tissues of the secondary wood as medullary 

 rays. A satisfactory appellation for them is simply wood rays, and 



